The invention relates to manganese dioxide useful as cathode active material in electrochemical cells, particularly alkaline cells. The invention also relates to an electrolysis process for preparing manganese dioxide having lower manufacturing cost.
Conventional alkaline electrochemical cells are formed of a cylindrical casing. The casing is initially formed with an enlarged open end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing forms the positive terminal.
The cell contents of a primary alkaline cell typically contain zinc anode active material, alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose. The anode active material comprises zinc particles admixed with conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, electrolyte and, optionally, some zinc oxide. The gelling agent holds the zinc particles in place and in contact with each other. A conductive metal nail, known as the anode current collector, is typically inserted into the anode active material. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cathode material is typically of manganese dioxide and normally includes small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight (dry basis). Electrolytic MnO2 (EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods.
EMD (electrolytic manganese dioxide) can be manufactured from the direct electrolysis of an aqueous bath of manganese sulfate and sulfuric acid. The EMD is a high purity, high density, gamma manganese dioxide, desirable as a cathode material for electrochemical cells particularly Zn/MnO2 alkaline cells, Zn-carbon and lithium/MnO2 cells. During the electrolysis process the gamma EMD is deposited directly on the anode which is typically made of titanium, lead, lead alloy, or graphite. The EMD is removed from the anode, crushed, ground, washed in water, neutralized by washing with dilute NaOH, Na2CO3, NH4OH or LiOH, and dried in a rotary dryer. The EMD product is generally heat treated to remove residual water before it is used in a lithium cell. Conventional electrolysis processes for the manufacture of EMD and a description of its properties appear in Batteries, edited by Karl V. Kordesch, Marcel Dekker, Inc. New York, Vol. 1 (1974), p.433-488. Conventional electrolysis processes for production of MnO2 are normally carried out at temperature between about 80 and 980xc2x0 C.
M. Mauthoor, A. W. Bryson, and F. K. Crudwell, Progress in Batteries and Battery Materials, Vol. 16 (1997), pp. 105-110 discloses an electrolysis method for manufacture of manganese dioxide. The electrolysis is performed at temperatures between 90 and 108xc2x0 C. Although Mauthoor reports that discharge capacities of MnO2 synthesized by electrolysis of an aqueous bath of MnSO4 and H2SO4 at between 95xc2x0 C. to 108xc2x0 C. was about 9% higher than that for MnO2 material produced at 95xc2x0C., there was no substantial difference among the three MnO2 products produced at electrolysis temperatures of 100xc2x0 C., 105xc2x0 C., and 108xc2x0 C. In fact, as Mauthoor increased the electrolysis temperature from 105 to 108xc2x0 C., the percent MnO2 in the electrolysis product and the discharge capacity of the MnO2 product both decreased slightly. Thus, electrolysis at temperatures higher than 108xc2x0 C. were not attempted or contemplated.
In commercial EMD production, the electrolysis is normally carried out at temperatures between 94xc2x0 C. and 97xc2x0 C. and at current densities between 2 and 10 Amp/ft2, more typically between 4 and 10 Amp/ft2 of anode surface area. A titanium anode and graphite or copper cathode are typically employed. Increasing current density tends to increase the MnO2 specific surface area (SSA). When electrolysis is carried out at conventional temperatures and current density is increased beyond the normal bounds, there is a tendency for the specific surface area (SSA) of the MnO2 product to increase to a level which is outside (greater than) the desired range of between 18-45 m2/g. Thus, at conventional temperatures it is very difficult to increase the current density and the deposition rate above a level of between about 10 to 11 Amp/ft2 (108 to 119 Amp/m2) without adversely affecting the quality of the product.
In addition, under conventional conditions of temperature and electrolyte composition, at current densities greater than 10 Amp/ft2 (108 Amp/m2) there is a tendency for passivation of the titanium anode to occur after a period of time, which may be shorter than the normal plating cycle of 1.5 to 3 weeks. The higher the current density, e.g. 12 Amp/ft2 (130 Amp/m2) rather than 10 Amp/ft2 (108 Amp/m2), the sooner such passivation is likely to occur. Passivation involves the formation of an insulating oxide film on the surface of the titanium, resulting in an increase in the operating Voltage of the anode. Once started the problem is self accelerating and soon results in a precipitous voltage rise which exceeds the capability of the power supply followed by a loss of current, ending in complete and irreversible shut-down of the plating process. Often a number of anodes will fail simultaneously due to passivation. When this occurs, the anodes must be withdrawn, deposited EMD removed and the anodes must be surface treated to remove the tenacious oxide film prior to being returned to service. This is a highly disruptive and expensive problem. In a commercial setting, great care is taken to avoid anode passivation and a margin of safety is preserved in setting the current density below that which borders on passivation, EMD quality considerations aside.
V. K. Nartey, L. Binder, and A. Huber, Journal of Power Sources, Vol. 87 (2000), p. 205-211 describes an electrolysis process for making MnO2 wherein the electrolysis bath was doped with TiOSO4. The MnO2 was used in an alkaline rechargeable battery. The reference states at page 210, col. 1 that the MnO2 with TiOSO4 doping (called M2, Table 7) performed poorly on the initial discharge cycle (i.e. similar to a primary, non-rechargeable cell) despite a high specific surface area. When the bath was doped with TiO2 the MnO2 product (called M1, Table 7) performed better on the initial discharge cycle, but still did not perform as well as the control MnO2 (commercial grade EMD Tosoh GH-S). The electrolysis bath for the experiments described in Huber, et al. was maintained at conventional temperature of 98xc2x0 C. and was performed at conventional current density of 6 milliAmp/cm2 (5.57 Amp/ft2) based on anode surface area.
Conventional battery grade manganese dioxide does not have a true stoichiometric formula MnO2, but is better represented by the formula MnOx, wherein x is typically between about 1.92 to 1.96, corresponding to a manganese valence of between about 3.84 and 3.92. Conventional EMD may typically have a value for x of about 1.95 or 1.96, corresponding to a manganese valence of 3.90 and 3.92, respectively. In addition to manganese (Mn) and oxygen (O), conventional electrolytic manganese dioxide (EMD) also contains a certain quantity of SO4=ions and of H30 ions (protons) in the crystal lattice. When heated to temperatures above 110 deg. C., the lattice protons combine with oxygen and are liberated as H2O. Conventional EMD also has a real density of between about 4.4 and 4.6 g/cm3.
There are increasing commercial demands to make primary alkaline cells better suited for high power application. Modern electronic devices such as cellular phones, digital cameras, toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices demand high power, for example, an AA cell may be required to deliver high power between about 0.5 and 2 Watt which corresponds to current drain rates between about 0.5 and 2 Amp, more usually between about 0.5 and 1.5 Amp. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications, without adversely affecting cell performance.
Accordingly it is desirable to extend the useful service life of electrochemical cells, particularly alkaline cells intended for high power applications.
An electrolysis unit is formed comprising an anode, a cathode, and an electrolysis bath comprising H2SO4 and MnSO4 dissolved in water. An aspect of the invention is directed to subjecting said electrolysis bath to electrolysis at elevated temperature and super atmospheric pressure while in said electrolysis unit. It has been determined that electrolysis at elevated temperature over 110xc2x0 C., preferably between 115xc2x0 C. and 155xc2x0 C., also advantageously between 120xc2x0 C. and 155xc2x0 C. and superatmospheric pressure is desirable in that it allows the electrolysis to be conducted at significantly higher current density (based on total anode surface) while maintaining the specific surface area of the product EMD in the desired range of 18 to 45 m2/g. The anode is typically of titanium. The cathode is typically of graphite. The electrolysis is carried out at elevated temperatures between about 110xc2x0 C. and 180xc2x0 C, preferably between about 115xc2x0 C. and 155xc2x0 C, also advantageously between 120xc2x0 C. and 155xc2x0 C. and corresponding superatmospheric vapor-liquid equilibrium pressure, or even at somewhat higher pressures. Specifically, conducting electrolysis at such elevated temperature, preferably between 115xc2x0 C. and 155xc2x0 C. and superatmospheric pressure allows higher current density of between 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2) more preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/M2), desirably between 18 and 30 Amp/ft2 (194 and 324 Amp/M2) based on anode surface area, to be employed while avoiding passivation of the titanium anode which typically occurs at very high current densities, e.g. greater than about 10 to 11 Amp/ft2 (108 and 119 Amp/m2). Essentially, electrolysis at temperature between 115xc2x0 C. and 155xc2x0 C. eliminates the problem of passivation of a titanium anode. (Passivation occurs as an insulating oxide film builds up on the anode.) It has been discovered that when the electrolysis is conducted at elevated temperature above 1150 C, e.g., between about 115xc2x0 C. and 155xc2x0 C. the problem of anode passivation of a titanium anode is essentially eliminated even if the current densities are increased to a level between 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2). By running the electrolysis at such higher current densities between 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2), preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/m2) of anode surface, a higher rate of production of MnO2 is achieved. That is, there is a significantly increased rate of MnO2 deposition on the anode (Kilograms MnO2 per square meter of anode surface per hour) This in turn means that the same rate of MnO2 production per hour can be obtained with smaller sized electrolysis units or else a fewer number of commercial sized electrolysis units can be employed. Therefore, capital investment in electrolysis equipment can be greatly reduced for the same MnO2 production rate per hour. The MnO2 produced under the electrolysis condition of elevated temperature between about 110xc2x0 C. and 180xc2x0 C., preferably between 115xc2x0 C. and 155xc2x0 C. coupled with increased current density between, 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2), preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/m2), desirably between 18 and 30 Amp/ft2 (194 and 324 Amp/m2) results in an MnO2 product (EMD) having satisfactory overall performance when used as cathode active material in alkaline cells.
One deleterious effect of conducting the electrolysis at the above stated elevated temperatures between 110xc2x0 C. and 180xc2x0 C., preferably between 115xc2x0 C. and 155xc2x0 C. and normal current densities (between 2 and 10 Amp/ft2 (21.5 and 108 Amp/m2) typically between 4 and 10 Amp/ft2 (43 and 108 Amp/m2) is that such operation has a tendency towards reducing the average specific surface area (SSA) of the MnO2 product. This can have a negative effect on overall performance of the MnO2 when used as cathode active material in an alkaline cell. It has been determined that such deleterious effect can be compensated for by a) increasing the current density to a level of between about 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2) preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/m2) desirably between 18 and 30 Amp/ft2 (194 and 324 Amp/m2) of anode surface or b) adding a soluble dopant, preferably a soluble titanium dopant to the electrolyte solution which is fed to the bath or to the bath itself. The most advantageous results are obtained when the electrolysis is conducted at both increased current density of between 12.5 and 37 Amp/ft2 (135 and 400 Amp/m2), preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/m2) desirably between 18 and 30 Amp/ft2 (194 and 324 Amp/m2) and a dopant, preferably a soluble titanium dopant, is also added to the electrolyte solution which is fed into the electrolysis bath or to the bath itself. In this latter case the resulting MnO2 product exhibits excellent overall performance when used as cathode material in an alkaline cell. Also, as above mentioned because of operation at higher current density, the capital investment in electrolysis equipment can be significantly reduced (because of the higher MnO2 deposition rate on the anode, smaller sized electrolysis units or fewer larger units can be employed). The doping agent or dopant is defined herein as an element, ion or compound that is soluble in the electrolysis bath or in the electrolyte solution which is fed into the electrolysis bath and has the effect of incrementally increasing the specific surface area (SSA) of the MnO2 product. Preferred dopants are soluble titanium salts such as TiOSO4, TiOCl2, CaTi2O4(OH)2, SrTiOF4 and (TiO)2P2P2O7. A preferred titanium salt for use as a dopant herein is TiOSO4.
It is desirable that the specific surface area (SSA) of the MnO2 product be between 18 and 45 m2/g. At specific surface area above 45 m2/g the individual crystallites become smaller and thus the number of individual crystallites within a fixed volume of MnO2 becomes greater. This leads to smaller pores between the individual crystallites that comprise the MnO2 particles. When the pores become too small, it becomes more difficult for the water molecules to enter therein and hydroxyl ions to leave when the MnO2 is used as cathode active material in an alkaline cell. This results in a decrease in reaction rate and poor alkaline battery performance. On the other hand when the specific surface area of the MnO2 product is much below 18 m2/g, this results in insufficient surface for the electrochemical reaction to occur as the MnO2 is discharged in an alkaline battery. Thus, it is desirable for the MnO2 product to have a specific surface area (SSA) between 18 and 45 m2/g.
It has been determined that since deposition of EMD at elevated temperature and super-atmospheric pressure tends to decrease specific surface area (SSA) and to favor the growth of larger, more perfect crystallites, a compensating adjustment or adjustments can be made to counter the effects of elevated temperature and return the product to a normal SSA (e.g. between 18-45 m2/g). It has been determined that such compensating adjustments which have the effect of increasing the specific surface area of the manganese dioxide product are a) operation of the electrolysis at higher current density, desirably between about 12.5 and 37 Amp/ft2 (135 and 400 Amp/M2), preferably between 18 and 37 Amp/ft2 (194 and 400 Amp/M2) of anode surface and b) adding a doping agent such as water soluble titanium dopant, e.g. TiOSO4 to the electrolyte solution which is fed to the electrolysis bath or to the electrolysis bath itself. It has been determined that the specific surface area of the manganese dioxide product can be desirably increased by conducting the electrolysis at said higher current density or adding the dopant. However, the most desirable overall effect is obtained when the electrolysis is conducted with both compensating adjustments being employed simultaneously. The use of the combination of these two adjustments readily elevates the specific surface area (SSA) of the MnO2 product to the desired range of between 18 and 45 m2/g. Additionally, the addition of dopant appears to cause some beneficial change on crystalline structure (primarily the gamma or gamma epsilon structure) of the MnO2 thus enhancing beneficial performance characteristics, e.g. higher open circuit voltage (OCV) and higher capacity, especially when used as cathode active material in an alkaline cell in high power application. The resulting MnO2 product exhibits excellent overall performance characteristics when used as active cathode material in an alkaline cell. Additionally, it has been determined that the resulting MnO2 product can have a slightly lower than normal real density. That is, the real density of the MnO2 product can be as low as between 4.2 and 4.38 g/cm3 . Such lower real density is thought to result from a higher level of crystal imperfections such as combined water, cation vacancies, twinning faults and [SO4]=anions which contribute to the higher OCV and the improved performance of the MnO2 when used as cathode active material in an alkaline cell.
The advantages of this procedure are that a superior quality EMD may be produced, particularly for high power applications and/or that the rate of production may be increased several fold (because of the electrolysis at higher current densities) over that of conventional EMD plating processes thereby reducing the needed investment in plating equipment and the size of the plant to accommodate this equipment. Labor is also reduced due to the shorter plating cycle and reduced time spent in monitoring the process for each unit of material plated.
The electrolysis bath desirably has an MnSO4 concentration of between about 0.2 and 2.0 moles/liter and the H2SO4 concentration between about 0.1 and 1.0 moles/liter. An anode and cathode are inserted into the electrolysis bath, and preferably submerged in the bath. The bath and electrodes are housed within a closed casing forming the electrolytic cell. The electrodes are connected to a direct current power source, which can be externally located. The anode is defined as the electrode at which oxidation occurs and therefore is connected to the positive terminal of the direct current power source and the cathode is connected to the negative terminal of the power source. During electrolysis the bath is maintained at an elevated temperature above 108xc2x0 C., desirably a temperature above 110xc2x0 C., preferably above 115xc2x0 C. Desirably the bath can be maintained at a temperature between about 110xc2x0 C. and 180xc2x0 C., desirably at a constant temperature between about 115xc2x0 C. and 155xc2x0 C., for example, between 120xc2x0 C. and 155xc2x0 C., typically at a temperature of 120xc2x0 C. The electrolysis is carried out at a current density between about 2.0 and 37.5 Amp/sq. ft (21.5 and 405 Amp/sq. meter) of anode surface. Desirably the current density is maintained at about 2.0 Amp/sq. ft. to 25.00 Amp/sq. ft. (21.5 Amp/m2 and 270 Amp/m2) of anode surface. Preferably, the electrolysis is carried out at elevated current density of between 12.5 and 37 Amp/ft2 (135 Amp/M2 and 400 Amp/M2) of anode surface. For comparison, conventional EMD electrolysis baths operating in the range of 80 to 98 deg. C. and normal atmospheric pressure normally operate at a current density of 5 to 8 A/ft2 and rarely exceed 10A/ft2 due to the well known danger of passivating the Ti electrodes used in the production of high quality EMD.
In conjunction with the above conditions of high temperature and super-atmospheric pressure and, optionally, increased current density, a doping agent, such as Ti+4, may be added to raise the SSA. For example, sufficient Ti+4 may be added to create a final doping level of about 2,160 ppm Ti in the final EMD deposit. This may be achieved by a variety of means such as adding a soluble titanium compound to the cell electrolyte (e.g. TiOSO4) or by corroding the Ti anode, prior to commencing the electrolysis, so as to dope the electrolyte with the required quantity of dissolved Ti, or by maintaining a body of Ti metal in the electrolysis cell throughout the electrolysis and inducing it to corrode at a controlled rate, so as to continually replenish the dissolved Ti, as it is consumed in the formation of the EMD deposit.
Preferably, hydrogen produced during the electrolysis is removed as effluent from the electrolytic cell along with at least a portion of spent electrolyte. The electrolysis bath can be continuously replenished with a flow of fresh electrolyte solution comprising fresh H2SO4 and MnSO4 in water. Thus, the aqueous electrolysis bath can desirably be maintained at a steady state composition, with the MnSO4 concentration at a constant value between about 0.2 and 2.0 moles/liter and the H2SO4 concentration at a constant value between about 0.1 and 1.0 moles/liter. Advantageously, the concentration of the MnSO4 in the aqueous bath is maintained at a steady state concentration of 0.9 mole per liter (136 g/liter) and the concentration of the H2SO4 at 0.5 mole per liter (49 g/liter).
During the electrolysis the cell is tightly sealed and a steady state gas phase is formed essentially in vapor-liquid equilibrium with the electrolysis bath. In a preferred aspect of the process, hydrogen produced during the electrolysis is continuously removed from the cell and thus its partial pressure in the gas phase can be very small and even negligible. In such case, the pressure maintained in the cell can essentially be equal to the water vapor equilibrium partial pressure corresponding to bath temperature at the bath steady state composition. The a cell during electrolysis is desirably maintained at a bath temperature, above 110xc2x0 C., preferably above 115xc2x0 C., desirably between about 115xc2x0 C. and 155xc2x0 C. and at a pressure approximating the vapor pressure of water in equilibrium with the electrolyte solution in the bath. For example, when the cell bath is maintained at a temperature of 120xc2x0 C., the cell pressure can be about 2 atmospheres absolute. Thus, if the cell bath is maintained at a constant temperature of between about 115xc2x0 C. and 155xc2x0 C., the cell equilibrium pressure can correspondingly be between about 1.7 and 5.4 atmospheres, absolute.
During the electrolysis, manganese dioxide accumulates continuously on the surface of the anode for around 1xc2xd to 3 weeks, until a thickness of about 1.0 to 3.0 cm is achieved. The electrolysis is interrupted at this point and the anodes are withdrawn from the bath. The MnO2 can be harvested by applying mechanical shock to the anode which fractures the MnO2 and allows it to fall away from the anode surface, as xe2x80x9cchipxe2x80x9d. The recovered manganese dioxide chip can be crushed, ground, washed, neutralized and dried in a conventional manner. The order of these operations may be interchanged, that is, grinding may be performed before or after washing etc. The manganese dioxide product of the invention has a predominantly gamma crystalline structure. However, the manganese dioxide product of this invention has a real density between about 4.20 and 4.40 g/cm3, more typically between about 4.20 and 4.38, for example, between about 4.20 and 4.35. Such real density range is lower than the real density of manganese dioxide made by conventional electrolysis processes, typically between about 4.4 and 4.6 g/cm3 when the electrolysis bath temperatures are maintained between about 80xc2x0 C. and 98xc2x0 C. It has been determined that the MnO2 product of the invention can have a real density between 4.20 and 4.40 g/cm3, more typically between 4.20 and 4.38 g/cm3 in conjunction (simultaneously) with a specific surface area (SSA) between the desired 18 and 45 m2/g.
The real density of a solid is the solid sample weight divided by its real volume, that is, the solids sample apparent volume reduced by the open pore volume occupied by air or liquid. The real solids volume is conventionally measured by displacement of He gas in a Helium Pycnometer apparatus. Prior to determining its volume, the sample is degassed by gentle heating in vacuum or in a dry gas stream, in order to eliminate gas or liquid residing in the open pores which might interfere with the measurement of the volume of solid material. Also the valence of the manganese in the manganese dioxide product of the invention is typically between about 3.94 and 3.98, which is higher than that achieved in conventional battery grade manganese dioxide including battery grade electrolytic manganese dioxide (EMD) made by conventional electrolysis methods. This corresponds to a stoichiometric formula MnOx, wherein x has an average value between about 1.97 and 1.99. Such higher valence is achieved by the electrolysis process of the invention without subjecting the manganese dioxide product to further oxidation, for example, without subjecting the manganese product to ozone gas, or other strong oxidizing agent to increase the oxidation state (valence) of the manganese. However, it should be understood that the manganese dioxide product of the invention could be subjected to such further treatment.
When the manganese dioxide of this invention contains doping elements or ions, particularly titanium and sulfate, it will exhibit open circuit voltage (OCV) greater than 1.65 V and more typically, 1.68 to 1.69 V when measured against a piece of clean, pure zinc metal in 9N KOH. Ordinary commercial manganese dioxide shows an OCV of 1.60-1.63 V and premium, high power manganese dioxide an OCV of 1.63-1.65 V under the same conditions.
The manganese dioxide product can be used as cathode active material in electrochemical cells, for example, zinc/MnO2 alkaline cells. It has been determined that when the manganese dioxide product containing doping elements or ions, particularly titanium and sulfate, is used as cathode material in alkaline cells, the service life increases substantially, on high power (0.5 to 2 watt) applications.